BALTIMORE (WJZ) — Opening arguments Wednesday in the case against a Baltimore City police officer accused of mistakenly shooting a police recruit in the head during a training exercise in February.

Derek Valcourt was in court as jurors heard from the the first witnesses.

Officer William Scott Kern, known to friends as Scott, never meant to shoot police trainee Raymond Gray in the forehead. But that’s exactly what happened as the police academy instructor taught a group of new recruits at the site of the old Rosewood Center for the developmentally disabled in Owings Mills.

A mistake that outraged even the mayor.

“There’s no acceptable explanation for why live rounds were at a training exercise,” Mayor Rawlings-Blake said.

Police trainee Derek Allen–on the site that day–was among the first to testify, describing Kern as armed with both his real gun and a similarly sized paintball style gun used for training.

Witnesses say Kern had his real gun holstered at his hip with his training weapon just inches below. Trainees testified that prior to the shooting, during a demonstration, Kern mistakenly pulled out the real gun at least once before.

Trainee Emiliana Korda told jurors after Kern mistakenly pulled out the gun the first time, she heard him say: “Oops. I’m sorry. That was muscle memory.”

A short time later, he made the same mistake. Only this time, pulling the trigger and striking Gray–now left with brain damage and blinded in one eye. Attorney Dwight Pettit represents Gray’s family in a civil suit.

“It seems to me he had to know which gun he had. I don’t see how that’s a mistake when you’ve had that kind of training,” Pettit said.

Late Wednesday, the prosecution rested. The defense will begin presenting their case Thursday. They plan to argue Kern needed to be armed with his real gun that day for the safety of all the trainees who were unarmed..

Kern faces charges of reckless endangerment and second-degree assault. If convicted, he could face up to 15 years in prison.